Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Consequently, the use of exhaust aftertreatment systems on engines to reduce emissions is increasing.
Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of carbon monoxide (CO), unburned hydrocarbons (UHC), diesel particulate matter (PM) such as ash and soot, and nitrogen oxides (NOx). Oxidation catalysts have been implemented in exhaust gas aftertreatment systems to oxidize at least some particulate matter in the exhaust stream and to reduce the unburned hydrocarbons and CO in the exhaust to less environmentally harmful compounds. To remove the particulate matter, a particulate matter (PM) filter is typically installed downstream from the oxidation catalyst or in conjunction with the oxidation catalyst. With regard to reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds.
SCR systems generate ammonia to reduce the NOx. When just the proper amount of ammonia is available at the SCR catalyst under the proper conditions, the ammonia is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if there is excess ammonia in the exhaust, ammonia can slip out the exhaust pipe. Ammonia is an extreme irritant and an undesirable emission. Accordingly, slips of even a few tens of ppm are problematic. Additionally, due to the undesirability of handling pure ammonia, many systems utilize an alternate compound such as urea, which vaporizes and decomposes to ammonia in the exhaust stream. Presently available SCR systems treat injected urea as injected ammonia, and do not account for the vaporization and hydrolysis of urea to component compounds such as ammonia and isocyanic acid. As a result, the urea can decompose to ammonia downstream of the SCR causing ammonia slip, and less ammonia may be available for NOx reduction than the control mechanism estimates causing higher NOx emissions at the tailpipe.
Also affecting the NOx reduction performance of SCR systems are the non-ammonia components of the exhaust gas entering the SCR catalyst (i.e., SCR feedgas). SCR systems are configured to reduce NOx to N2 and H2O in the presence of ammonia (NH3). The most efficient chemical reaction for reducing NOx requires an optimal level of nitrogen dioxide (NO2) relative to nitrogen monoxide (NO). Accordingly, the NOx reduction performance of an SCR system is based on the relative ratio of NO2 and NO. Generally, if the relative amount of NO2 in the SCR feedgas is low, the overall NOx reduction performance of the SCR system is correspondingly low, and the tailpipe outlet NOx is unacceptably high. Therefore, the amount of NO2 in the SCR feedgas is a focus of most conventional engine systems with SCR components.
The level of NO2 in the SCR feedgas is at least partially controlled by the oxidation catalyst (e.g., a diesel oxidation catalyst) downstream of the internal combustion engine and upstream of the SCR system. The oxidation catalyst is made from catalytic materials that promote the catalytic oxidation of NO into NO2. Accordingly, the oxidation catalyst converts NO in the exhaust gas generated by the engine into NO2. In this manner, the performance of the oxidation catalyst is tied to the NOx reduction performance of the SCR system.
The performance of a PM filter (e.g., a diesel particulate matter filter) upstream of the SCR system may also affect the NOx reduction performance of the SCR system. For example, some PM filters also oxidize NO to form NO2 independent of the oxidation catalyst. Should the PM filter fail to properly convert NO to NO2, the level of NO2 in the SCR feedgas entering the SCR system will be adversely affected. Accordingly, the performance of the PM filter also is tied to the NOx reduction performance of the SCR system.
Current emissions regulations require the diagnostic monitoring of exhaust aftertreatment components and on-board reporting of the performance of the components in substantially real time. More specifically, current on-board diagnostic (OBD) systems must alert a vehicle operator when one or more components of an exhaust aftertreatment system are malfunctioning or performing inadequately.